The present application claims priority under 35 U.S.C. xc2xa7119 of German Patent Application No. 199 44 267.3, filed on Sep. 15, 1999, the disclosure of which is expressly incorporated by reference herein in its entirety.
1. Field of the Invention
The invention relates to a process for drying of a material web, in particular a paper or a cardboard web. The invention further relates to a device for drying a material web as well as a drying section of a machine for producing a material web.
2. Discussion of Background Information
In conventional multi-cylinder drying sections, the material web to be dried is guided over a steam heated cylinder or over an arrangement of steam heated cylinders and wire suction rolls. Especially at the beginning of drying, problems occur in the guidance of the web which are caused, in particular, by the fact that the still-moist material web, not yet having sufficient consistency, adheres to the smooth contact surfaces that are necessary for sufficient heat conductivity. This often leads to web breaks and to an over stretching of the web seams. Coating the cylinders can reduce these problems only to an insufficient degree. This results in the necessity of slowing the drying process, which lengthens the necessary drying section. The problems mentioned above occur increasingly at higher web travel speeds.
The present invention to create a possibility for drying material webs as fast as possible in secured guidance and at web travel speeds as high as possible.
In particular, the present invention provides for the material web to be introduced to at least one heated area and to remain in uninterrupted contact with the heated area until the web achieves a sufficient consistency be lifted off the heated area.
According to the invention, the separation of the material web from the heated area is omitted as long as the material web is not firm enough to be lifted off without any problemsxe2x80x94and in particular without adhering to the heated area. In contrast to multi-cylinder drying groups of single or multiple rows, in which the material web is only briefly in contact with the separate drying cylinders and is set successively onto an area and is lifted off an area in short intervals, according to the invention, the material web remains longer in contact with the heated area so that the material web can solidify without the interruptions of being set down onto and being lifted off from an area.
According to the invention, the material web to be dried is intentionally guided along a detour, i.e., the distance to be traveled by the material web inside of the respective entire arrangement is extended by the heated area in order to insert in this manner a heating distance with the necessary length into the machine.
A considerable advantage of the invention includes the fact that a smooth surface, and in particular a smooth metal surface, can be provided as the heated area that enables the optimal transmission of heat onto the material web and thus an efficient web drying. Another advantage of the invention is the fact that, by providing a heated area which is brought into contact with the material web to be dried, additional heating elements for influencing the material web with heat are not necessary.
Furthermore, due to the uninterrupted contact of the material web with the heated surface, it is not necessary to take special measures, such as, e.g., providing wire belts between heated areas and the material web in order to avoid adhesion of the material web to the heated area. According to the invention, the material web is lifted off only when, due to the relatively high firmness of the material web, the chance of a break is no longer present or at least drastically reduced. Because, according to the invention, the material web is already sufficiently firm and/or dry at the time of the separation, the lifting of the material web can occur at comparatively high web travel speeds. The invention therefore enables not only a fast drying due to the contact between the material web and the heated area, it also allows drying of the web to be performed at high machine speeds.
The invention is used particularly advantageously in producing paper or cardboard webs. Here, the heated area can be provided immediately behind the press arrangement from which the still-moist material web exits, for instance, with a dry content of about 45% to 55%. According to the invention, the material web can be dried on the heated area to such an extent that the lifting of the material web occurs at a dry matter content of, for instance, about 55-65%. In general, however, it is true that the heated area is intended to cause an increase in the dry matter content of at least 1%, preferably at least 2%, in particular at least 4%. The material web which thus already has a sufficient consistency can then be introduced into a subsequent conventional multi-cylinder drying group. The chance of adhesion onto one of the separate cylinders of this drying group no longer exists since the material web is already sufficiently dry. The multi-cylinder drying group can therefore be designed very simply, since special measures for guidance of a not yet sufficiently hardened material web are not necessary.
According to a preferred embodiment of the invention, the jacket surface of the cylinder is provided as the heated area. Here, the maximal length of the heated area is determined by the diameter of the cylinder which is selected to be preferably considerably larger than the diameter of drying cylinders that are used commonly in drying groups of one or more rows in machines for producing paper or cardboard webs. The diameter of the cylinder can, for instance, be twice as large as that of conventional drying cylinders. In general, the large cylinder can also be several times larger than a comparable conventional drying cylinder.
The size of the cylinder is determined in such a way that, in particular dependent on the web travel speed and the drying ratio that can be achieved with the heated surface, a heating zone is provided between the feeding zone in which the material web is fed to the heating area and is set onto the jacket area of the cylinder and a removal zone in which the material web is lifted off the heated area, which heating zone is long enough in the web travel direction to ensure a sufficiently high firmness of the material web in the removal zone.
According to an alternative embodiment of the invention, the heated area is provided in the form of the outer surface of a continuous belt that is guided around at least two idle rolls. Preferably, only two rolls are provided so that the continuous belt can be guided like a conveyer. By changing the distance between the two idle rolls, the length of the heated area can be varied. In this manner, an adjustment can occur to the desired web travel or machine speed or to other characters of the material web and/or to the producing machine. In general, it is possible, according to the invention, to guide the continuous belt around more than two idle rolls.
According to another embodiment of the invention, the material web is kept in contact with the heated area by a permeable belt, preferably a wire belt, and in particular is pressed against the heated area. Preferably, a drying mesh is used which is free of any markings and is sufficiently open to enable the evaporation of the moisture contained in the material web. When using a large cylinder, it can also be covered separately to favorably influence the removal of the material web by a possible speed difference of a suction roll provided for the removal of the material web
According to another embodiment of the invention, the material web is adhered to the heated area. Here, the material web is therefore not kept in contact with the heated area by special devices such as, e.g., wire belts. The adhesion of the material web occurs preferably by a pressure roll which can originate directly from a pressure felt of a preceding press arrangement below or above the heated area. For the guidance of the material web from the press arrangement to the heated area an additional feeding belt can be provided, for instance, a transfer felt, transfer wire, or transfer belt.
A device for drying a material web as well as a drying section of a machine for producing a material web is also protected by the invention.
The present invention is directed to a process for drying a material web. The process includes guiding the material web to at least one heated area, maintaining the material web in uninterrupted contact with the at least one heated area at least until the material web achieves a firmness sufficient for detaching the material web from the heated area.
In accordance with a feature of the instant invention, the material web can include one of a paper and a cardboard web.
According to another feature of the present invention, the material web can be positioned directly onto the heated area. Further, no intermediate layers of one or more wire belts are located between the material web and the heated area.
The at least one heated surface can include a smooth surface. Further, the smooth surface can include a smooth metal surface.
In accordance with another feature of the invention, the at least one heated area may include a continuous belt.
Further, the process can include guiding the material web from the at least one heated area to at least one multi-cylinder dryer group. A length of the contact between the material web and the at least one heated area can be larger than a circumference of the dryer cylinder which comprise the at least one multi-cylinder dryer group. The length of the contact between the material web and the at least one heated area may be considerably larger than a circumference of the dryer cylinder which comprise the at least one multi-cylinder dryer group.
Moreover, the at least one heated area may include a jacket surface of a cylinder. A diameter of the cylinder can be between about 4-10 meters. In an area of the cylinder jacket, the cylinder may include several layers. The cylinder jacket may include a circulating continuous belt. Further, the process can include heating a jacket area of the cylinder from the inside. The heating can be provided by at least one of from the inside of the cylinder by a hot fluid and from the outside by at least one of inductively and by IR-rays.
According to another feature of the invention, the at least one heated area may include the outer surface of a continuous belt guided around two idle rolls.
An inner surface of the continuous belt may be heated at least partially by at least one heating unit which is preferably positioned inside a loop formed by the continuous belt. A distance between the two idle rolls may be a multiple of a diameter of the idle rolls. One of the idle rolls can be positioned in a space between a press arrangement and a multi-cylinder dryer group, and the continuous belt can be arranged in the space to extend diagonally downwardly.
The process can further include protecting the material web in contact with the at least one heated area from falling objects. The material web may be protected from objects falling from a multi-cylinder drying group. The material web can be protected by a removal device for the material web. The removal device may include at least one transfer wire arranged to pick-up the material web from the continuous belt.
The material web can be kept in contact with the at least one heated area by a permeable belt. The permeable belt may include a wire belt. The permeable belt may be arranged to press against the at least one heated area.
The process may further include adhering the material web to the at least one heated area via a press roll.
In accordance with a further feature of the present invention, the process can include detaching the material web from the at least one heated area by a suction roll.
According to another aspect of the instant invention, the process can include exposing the material web in contact with the at least one heated area to a hot fluid comprising at least one of hot air and hot steam.
Further, the process may include detaching the material web from the heated area when a dry content of the material web is at least 1% higher than in the beginning of the heated area. The dry content can be at least 2%, and the dry content may preferably be at least 4%.
Moreover, the material web, when it is set onto the at least one heated area, may have a dry content of between about 45-55%, and the process can further include detaching the material web from the at least one heated area with a dry content of between about 55%-65%.
According to still another feature of the instant invention, the heated area can be provided at least one of immediately behind a press arrangement, immediately in front of a multi-cylinder drying group of a machine for producing a material web, and immediately in front of another drying unit. The another drying unit can include a drying cylinder.
The present invention is directed to an apparatus for drying a material web, e.g., one of a paper and a cardboard web. The apparatus includes at least one heated area arranged to receive the material web. The at least one heated area includes a removal zone arranged for the removal of the material web from the at least one heated area. The at least one heated area has a length is sufficient to achieve a sufficient firmness of the material web to be removed in the removal zone.
According to a feature of the invention, the at least one heated area may include a cylinder having a diameter of between about 4-10 meters and a wire belt may be arranged to hold the material web onto a surface of the cylinder. Guide rolls can be arranged to guide the wire belt around the cylinder. The cylinder may be heatable from at least one of the inside and the outside.
In accordance with yet another feature of the invention, the at least one heated area can include an endless conveyor arranged to transport the material web from a press device to a dryer group. Heating units can be arranged within a loop formed by said endless conveyor.
The present invention is directed to a dryer section of an apparatus for drying a material web. The dryer section includes at least one heated area arranged to receive the material web. The at least one heated area includes a removal zone arranged for the removal of the material web from the at least one heated area. The at least one heated area has a length is sufficient to achieve a sufficient firmness of the material web to be removed in the removal zone.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing.